Safari users are more likely to see the biggest updates. You'll find Dark Mode support for those sites that allow custom color schemes, and you should see a slicker web login process when you're using Password AutoFill. Security and privacy have improved with warnings for insecure pages as well as a replacement for the old Do Not Track standard with Intelligent Tracking Prevention that bars cross-site tracking as a matter of course. And if you hate getting pop-ups asking you about website push notifications the moment you visit, you're in luck -- Safari will only allow those prompts after you've interacted with the site.
Other upgrades are subtler. Macs now support second-generation AirPods, and Maps includes the Air Quality Index in the US, UK and India. You'll get real-time text for phone calls you make through a nearby iPhone. You'll also find a host of bug fixes that touch on App Store updates, USB audio, MacBook Air screen brightness, Mac mini graphics and WiFi connection problems, among others.